FIG. 1 shows a U-shaped heat pipe pressed. The heat pipe 1a has an evaporation section 10a. The bottom of the evaporation section 10a must be pressed to form a flat heated surface 100a for directly and planarly touching a heat source. During the pressing process, the stamping die must have a flat plane. When a plane of the stamping die initially meets a curved surface of the heat pipe 1a, the touch portion will be linear and then become planar. However, the initially linear touch tends to invite a problem of stress concentration. Therefore, a recess 101a often forms on the heated surface 100a of the heat pipe 1a. When once the recess 101a appears, an additional grinding procedure after pressing will be necessary for effacing the recess 101a. 
This problem can be solved by adopting a multi-stroke progressive pressing procedure. This procedure can progressively press the pipe to be flat, but it must use various stamping dies with different recessing depth or shapes. Meanwhile, only one stamping die can be used to press the pipe at some time. Thus, during the pressing process those stamping dies must be changed one by one in order to ensure the flatness of the pipe being pressed. It is very inconvenient and uneconomical for the manufactures.